The present invention relates to the field of inspecting the machining of mechanical parts, and more particularly it relates to frequency inspection of one-piece bladed wheels by vibro-acoustic signature.
Until recently, the rotary portion or xe2x80x9crotorxe2x80x9d of the compressor in a turbojet or a turbo-thruster (referred to below as a xe2x80x9cturbomachinexe2x80x9d), has been built up by assembling together a plurality of wheels in the form of disks or rings, with each wheel having blades, also known as xe2x80x9cvanesxe2x80x9d, individually attached thereto. One consequence of individually mounting blades in that way is to cause vibratory phenomena to be damped, thereby making it impossible, in practice, for resonance to destroy a wheel on which blades are mounted.
Nowadays, compressor wheels, and also industrial fan wheels, are being made more and more frequently as single pieces, with the blades being integral with the wheel disk or support ring. That type of structure serves to minimize the size and the weight of the wheel and thus also of the compressor as a whole, which is beneficial for the overall mass of the turbomachine that incorporates it.
Unfortunately, modern means for automatic machining enable very great precision and very great repeatability to be achieved in the manufacture of blades, and an unexpected consequence of an almost perfect shape being made is to provoke very large resonant vibratory phenomena which can, under extreme circumstances, lead to the wheel being completely destroyed.
An object of the present invention is to provide a method-and a corresponding apparatus for acoustic inspection of one-piece bladed wheels that enable this risk of resonance to be identified, in order to avoid potential destruction of the wheel. Another object of the invention is to propose an inspection method which is particularly fast and which can be implemented in real time. Another object of the invention is to propose an inspection method which can form an integral portion of the machining process. Yet another object of the invention is to propose a method which also makes it possible to inspect the quality of the machining. A further object of the invention is to propose a method which can also determine the types of defect that affect any blades found to be defective.
These objects are achieved by a method of acoustically inspecting a one-piece bladed wheel in which the wheel is driven in rotation; each blade of the wheel is subjected to mechanical excitation; its acoustic response is picked up and a corresponding electrical signal is generated; its frequency response is determined by computing a fast Fourier transform (FFT); the electrical signal and the associated frequency response are stored; the characteristic frequencies of each blade of the wheel are identified; and a wheel is rejected or accepted depending on whether or not the frequency distribution obtained in this way matches a predetermined set of forbidden frequency distributions.
With this particular method, it is possible for a wheel that is not compliant to be identified very quickly and without error. Furthermore, the method can be integrated without difficulty in a conventional machining process.
Preferably, the method also includes an additional step of determining the defects of a blade by comparing its frequency response with predetermined frequency responses that are characteristic of various types of defect.
The invention also provides apparatus implementing the above method. Advantageously, the mechanical excitation means of the apparatus comprise means for excitation by percussion or by releasing a hammer, and the acoustic receiver means comprise a microphone.